This invention relates to a power matching network for use in a high frequency power amplifier circuit in which at least one semiconductor device is used as an active circuit element. In other words, the power matching network is for use in cooperation with an amplifier which comprises the semiconductor device and is herein called a transistor amplifier.
The high frequency power amplifier circuit is typically a microwave power amplifier module comprising a transistor. The transistor amplifier produces at its output end or terminal an amplified signal which comprises a fundamental frequency signal and odd and even harmonic signals.
It is already known that such a microwave power amplifier module is given a high amplifier efficiency when put in Class B operation together with use of a high frequency signal processing circuit, namely, a high frequency trap for adjusting production of the harmonic signals. A theoretically highest amplifier efficiency of 100% is achieved with Class F operation in which a load of the transistor amplifier is matched in connection with the fundamental frequency signal, is shorted or short circuited for all even harmonic signals, and is open ended as regards all odd harmonic signals. This fact is clear from a study of an article contributed by David M. Snider to the IEEE Transactions on Electron Devices, Volume ED-14, No. 12 (December 1967), pages 851 to 857, under the title of "A Theoretical Analysis and Experimental Confirmation of the Optimally Loaded and Overdriven RF Power Amplifier".
In practice, implementation of the Class F operation is troublesome because of possible presence of an infinite number of odd and even harmonic signals. Various power matching networks are consequently in use in the manner which will later be exemplified. For example, power matching networks are disclosed in Japanese Patent Prepublications (A) Nos. 44,611 of 1989 and 220,503 of 1990.
Such a conventional power matching network is appreciably excellently operable. The network is, however, defective in some respects. By way of example, the conventional power matching network is bulky. This is objectionable on using the network in a compact device, such as a portable radio telephone set. Furthermore, the network can not be correctly operable either as an open circuit or as a shorted circuit for some of the harmonic signals.